Document image scanners typically include roller systems forming a transport path for a document fed into the scanner to move the document past imaging devices, such as a linear CCD (charged-coupled device) camera. The roller systems generally include multiple rollers, driven by a single motor or multiple motors, which serve to move the documents along the transport path within the scanner. Typically, when a stack of documents are fed into the entrance of a scanner, they first encounter an urging roller which urges a stack of documents to an adjacent feed roller, which mates with a separation device. The separation device provides a recessive force to the lead edge of a stack of documents and separates a single document from the stack and then advances the single document to transport rollers and past the camera.
However, documents fed through the scanner may cause the different rollers to turn at different velocities. This can lead to transport slippage, misfeeds, and a reduction in image quality. For example, when the trailing edge of a document leaves the separation device the scanner transport may speed up slightly due to the reduction in load on the drive system. These changes in transport speed cause the document to move past the cameras at different speeds, resulting in reductions in image quality. In typical document image scanners which utilize a linear CCD camera, a constant velocity of the document being imaged past the camera is desired, to achieve the best image quality.
What is needed is a system and method to monitor the velocity of the transport system, and in particular the velocity of the rollers within the system. The velocity of various rollers within the system may then be controlled at various times, such as when a document is entering or leaving a new set of rollers, in order to reduce slippage, jams, and improve image quality.